Silicone-in-water emulsions of high viscosity silicones are easier to handle in certain applications than non-emulsified silicones. Thus, emulsification of such silicones enables their mixing with water-soluble ingredients or other emulsions. Silicone emulsions are well-known in the art and are widely used in coating, household, medical, cosmetic and personal care applications in order to provide, among other things, greater durability, protective qualities, water resistance, and barrier properties.
Silicone-in-water emulsions are typically made by mechanical emulsification, mechanical emulsification by inversion, or emulsion polymerization. Emulsions of low viscosity silicones are typically made by mechanical processes. High viscosity silicone emulsions can also be made by mechanical processes. For example, methods involving mixing surfactants with a silicone gum, adding water to form a water-in-oil emulsion, and applying high shear to cause inversion to an oil-in-water emulsion are known. However, because silicones are highly hydrophobic, because mechanical mixing becomes more difficult as the viscosity of the silicone component increases, and because mechanical mixing of highly viscous components can result in uneven mixing, mechanical processes have generally been found to be unsuitable for making emulsions of high viscosity silicones. Therefore, preparation of silicone-in-water emulsions of high viscosity silicones has for all practical purposes been limited to emulsion polymerization processes.
In a conventional emulsion polymerization process, a siloxane is emulsified before it is polymerized. The process typically occurs in three stages (i) formation of micelle particles and droplets of siloxane (generally polyorganosiloxanes), (ii) diffusion of the siloxane to the micelle particles where polymerization is initiated, and (iii) growth of the polymer until a desired molecular weight is achieved or the siloxane is consumed. The polymerization reaction occurs at the silicone/water interface, where the rate of polymerization is faster with smaller particles because the droplet surface area to droplet volume ratio is higher, which allows for faster transport mechanisms and thus, higher reaction rates. Disadvantages of such conventional processes include, but are not limited to, a limited range of viscosity for the internal phase (for example, high viscosity polymers are particularly problematic), a long batch time (and thus, increased production costs), limited concentration of silicone and formation of undesirable levels of byproducts (for example, cyclosiloxanes).
There have been various attempts to overcome these and other disadvantages. For example, processes have been described involving feeding siloxanes, catalysts for initiating polymerization, surfactant, and water through a series of mixers, extruders, or combinations thereof, wherein polymerization of the siloxanes occurs during or after emulsification. However, while such processes attempt to control viscosity and molecular weight of the silicones, such control is not optimal. Additionally, because such processes generally are not continuous there are inefficiencies due to reliance on various pieces of equipment.
Continuous and quasi-continuous processes for preparing silicone oil-in-water emulsions have been attempted wherein siloxanes undergo polymerization in the presence of a solvent and/or other inert fluid, followed by addition of surfactant, water, and shear for emulsion formation. However, the solvents required by such processes are retained within the silicone polymers and are therefore incorporated in the internal phase (i.e. silicone polymer) of the emulsions.
Reducing the presence of solvents, unreacted siloxanes, catalyst residues, cyclic polymerization byproducts, and other impurities in silicone emulsions is an ongoing challenge in the art. The need to reduce such impurities may arise, among other reasons, when the presence of impurities is incompatible with downstream applications (for example, medical, cosmetic, and personal care applications), where the presence of impurities would reduce the stability of an emulsion, or where regulatory requirements require removal or reduction of their presence. In particular, it is desirable to reduce the presence of cyclosiloxanes, such as octamethylcyclotetrasiloxanes and decamethylcyclopentasiloxanes, in silicone emulsions.
Thus, there remains a need in the art for a continuous process for preparing high viscosity silicone-in-water emulsions, whereby the particle size, viscosity, silicone concentration (i.e. % solids), and molecular weight of the silicone polymer can be optimally controlled, whereby solvents and other impurities (especially cyclosiloxanes) are minimized, and whereby efficiency is achieved through the use of a single apparatus.